1. Field of the Invention
The present invention relates to image processing, and, more particularly, to a channel for data transmission in, or watermarking of, video signals.
2. Description of the Related Art
Image information is commonly processed, recorded, and/or stored in either analog or digital format, but ultimately any information recorded in an analog format may be translated into a digital format. Once in digital format, such information is easily copied, modified, or distributed through electronic media such as the Internet. While protection by copyright may be available to owners of the information once fixed in tangible media, theft through unauthorized copying, modification, or distribution of the information when it is in digital format is 1) easily accomplished and 2) difficult to detect from the information itself. For example, theft of compressed digital content (e.g., digital videodisk (DVD), digital broadcast, or digital cinema) is a serious concern to owners and providers authorized to distribute the compressed digital content. While the digital format allows for xe2x80x9cperfectxe2x80x9d copies of the compressed digital content, techniques exist to hide information within the compressed digital content that may be used to prove rightful ownership or use, such as in a court of law.
Collectively, these techniques are within the field of data verification, and for encoded and/or compressed digital image content, digital watermarking is commonly employed to insert hidden information directly into the image video signal. Many watermarking techniques for digital images have been developed that allow content providers to prove the origin of their material and that their material was copied, modified, or distributed. In addition, published techniques exist for inserting invisible watermarks into video data, and the Copy Protection Technical Working Group (CPTWG) of the DVD forum, for instance, may select a standardized digital watermarking technique for DVD""s.
For some systems, a separate data channel superimposed on the transmitted video signal may aid in data verification. The data channel may be added within the encoded video signal data stream itself, or the data channel may be sent separately in secondary channels used for system control, signaling, and/or synchronization information. The channel may be used to carry data corresponding to an encryption key, watermark pattern, or other form of identifier unique to an authorized owner or distributor of the image content.
Some watermarking techniques exploit a human""s insensitivity to high spatial and/or temporal frequencies, and insert low-level watermarks with these characteristics. As is known in the art, both electronic displays and film projectors tend to have display characteristics that vary from the center to the sides of the image. However, a human""s visual perception is such that most humans observing the image tolerate and accept these subtle variations. Because the changes in the display characteristic(s) are slight, most viewers are not aware when a display characteristic changes over time.
In many of the techniques for encoded or compressed digital images, the watermark is xe2x80x9cstampedxe2x80x9d into the image by modifying either the (analog) pixels themselves, or on selected bit values of the (digital) pixel color component values (e.g., the red, green, and blue (RGB) components). Watermarks inserted with such techniques are generally degraded or destroyed by common image signal processing methods that are applied to the digital image information, such as scaling, cropping, rotation, and compression. Furthermore, techniques for relatively high bit-rate watermarking tend to be more visible in the watermarked image and more susceptible to subsequent signal processing.
Unfortunately, the watermarks of these techniques tend to be degraded or obliterated completely when the image is resized (specifically, reduced in size) or compressed by commonly used algorithms such as those specified in the MPEG-1 or MPEG-2 standards. Furthermore, the rapid variations associated with the watermark are often interpreted as noise by signal processing algorithms compressing the image data, which tends to lower the efficiency of compression algorithms. Other pixel-domain watermarking techniques specifically target edges of objects in the image as a means for inserting a watermark or hiding data. However, since data compression algorithms such as those employed in MPEG-2 systems introduce noise at object edges, a similar degradation or obliteration of the watermark may occur.
The present invention relates to a channel allowing for watermarking of an image or data transmission within an image. The presence of the channel itself may be considered as the watermark, or the watermark may be represented by data carried in the channel. The channel is inserted into the image by varying display characteristics of the image across the display in accordance with a predefined spatio-temporal pattern. Variation of a display characteristic for preferred embodiments is based on a predefined spatio-temporal pattern within pixel pairs, pixel lines, pixel columns, or pixel diagonals of an image frame. For some embodiments the spatio-temporal pattern and a corresponding, complementary pattern are applied to corresponding successive pairs of such frames.
In one exemplary embodiment, a watermark is added to a video signal representing an image. The watermark is added by (a) applying a first watermark function to a first set of pixels in a first frame; and (b) applying a complement of the first watermark function to a second set of pixels in the first frame. In a further embodiment the relative positioning of the first and second sets of pixels tends to mask visibility of the watermark in the first frame.
In another exemplary embodiment, a watermark is detected in a video signal representing an image. The watermark is detected by (a) identifying first and second sets of pixels in a first frame; (b) calculating a first difference value for the first and second sets of pixels; and (c) determining whether the watermark is present based on the first difference value. When the watermark is present, the first difference value is related to prior application of a first watermark function to the first set of pixels and prior application of a complement of the first watermark function to the second set of pixels. In a further embodiment relative positioning of the first and second sets of pixels tends to mask visibility of the watermark in the first frame.